This invention relates generally to semiconductor light sources, and, more particularly to multiple wavelength, broadly tunable optically pumped semiconductor diodes and lasers.
In a standard semiconductor laser or a light emitting diode, recombination between electrons in the conduction band and holes in the valence band produces photons that are emitted. The wavelength of the emitted photons is determined by the transition energy between the two states. The active region of the device, where the recombination takes place, can be either relatively thick, or of quantum dimensions (e.g., a quantum well) in order to facilitate this process. The energy of the transition can be modified either by changing the composition of the material in thick active-layers, or by changing either the thickness or composition of the quantum wells. FIG. 1 gives a schematic example of a quantum well laser in which the two fundamental levels within the quantum well located inside the cavity of the laser (indicated by E1 and HH1) determine the emission wavelength.
However, if two or more quantum wells emitting at different wavelengths are placed in the cavity of a standard semiconductor diode laser (FIG. 2), the laser will not emit at more than one wavelength simultaneously. In fact, competition between the different wells will occur, one will prevail, and the device will emit solely at the wavelength of that well. The reasons for this are well known. Briefly, as the injection current is increased, wells begin to fill with carriers but the laser threshold is first crossed for only one of these wells. Once this happens, the device is in laser operation mode with stimulated emission. Stimulated emission redirects the totality of the injected carriers (electrons and holes) to the lasing well at which they are promptly consumed. The Fermi level (a measure of carrier density) is therefore blocked and no longer increases when the injection current is increased. The other wells consequently cannot further increase their carrier density and will not reach their lasing threshold.
A less common type of semiconductor laser is one where carriers are generated by optical pumping, rather than electrical injection. These lasers are referred to as optically pumped semiconductor lasers (OPSLxe2x80x3s). In this case, the pump light is absorbed within the semiconductor structure and generates carriers, which in-turn diffuse toward the active layers (thick or quantum-sized) where they recombine to emit photons at a different wavelength. Despite the different method of generating carriers as compared to the standard diode laser, the carriers distribute themselves in a similar manner. In the case of multiple quantum wells emitting light at different wavelengths, one quantum well will prevail and disallow the others from reaching a lasing threshold.
Techniques that are external to the laser device, such as an external cavity with a grating, can force a laser to emit at a particular wavelength. These external techniques can therefore be used to select the emission wavelength of a laser from the range of wavelengths that the active regions are capable of producing (i.e., its bandwidth). Semiconductor lasers containing multiple wells in the same laser cavity have been successful in producing lasers with en extended bandwidth. However, simultaneous emission of more than one wavelength from the same cavity of a semiconductor laser has not been accomplished using either a standard diode design or an optically pumped design.
Inter-subband quantum cascade lasers (QCL""s), which appeared in 1994, are not considered to be standard semiconductor diode lasers, They are capable of producing lasers that emit at more than one wavelength. They differ from standard lasers in that the emission of a photon occurs as a result of transitions within only one band and therefore involves only one type of carrier. There is no disappearance of carriers during the emission of a photon as is the case with standard lasers (see U.S. Pat. No. 6,091,751). The same electron may furthermore be used several times in several quantum wells in crossing the structure, and may thus emit several photons. In its path crossing the structure, the electron can be forced to undergo transitions that are not all equivalent in energy. Because stimulated emission at one wavelength does not preclude reaching stimulated emission at another wavelength, two or more wavelengths can, in principle, be emitted simultaneously from such lasers. Moreover, QCL""s also allow the population of additional electronic levels within the conduction band to produce simultaneous lasing emission at more than one wavelength. The drawback of manufacturing multi-wavelength lasers using QCL""s include the fact that the different wavelengths tend to be closely separated, not independently engineered, and require highly complicated epitaxial structures.
Accordingly, there is a need for an OPSL that can simultaneously lase at a plurality of wavelengths from the same cavity or emit within a broadly tunable wavelength range.
In a preferred embodiment, an optically pumped semiconductor laser capable of simultaneously emitting a plurality of wavelengths is disclosed. The pump wavelength absorbing region is subdivided into n+1 regions by n partition layers. The partition layers are composed of a material that is transparent to the pump wavelength, but an electronic barrier to carrier diffusion and epitaxially compatible with other layers of the semiconductor laser structure. One or more essentially identical quantum well structures are located within each subdivided region of the absorbing region. The quantum well structures of at least one the subdivided regions is fabricated to emit at a different wavelength than any other subdivided region, so that two or more wavelengths may be simultaneously emitted. The number of essentially identical quantum wells within a given subdivided region can be varied to vary the relative intensity of its emitted wavelength. Or, the relative intensity of the emitted wavelengths can be varied by increasing or decreasing the volume of the absorbing medium by the positioning of the partition layers.